I know this subject comes up, and every time the response is use imapsync. Well, I don't see that as a workable response.
We are moving to a hosted Zimbra solution. We have 3.4TB of maildirs (~35000 users) to migrate. And, it needs to be done in over the Xmas weekend.
We built them special IMAP servers (using a pair of Sun X4150s), and set all the passwords to the same....separate from our normal IMAP serverfleet (4 X4100s [webmail & pine/pop3 users] and 4 v20z's [imap client users]).
Right now they are trying to migrate pre-pilot group of a dozen email accounts, and I heard they are estimating it'll take 30 days to do it. The special IMAP servers are bored....and our Internet connection is not being saturated.
We need a faster way to migrate.